In uplink HSPA (High-Speed Packet Access) all users are transmitting using same spreading codes and users are separated by non-orthogonal scrambling codes. This results that users in uplink HSPA transmissions need to share an interference limited resource. Reducing the interference in uplink frequencies is thereby critical to ensure high SIR (Signal-to-Interference Ratio) levels on uplink transmission channel and to maintain a stable system operation.
The HS-DPCCH (High-Speed Dedicated Physical Control Channel) transmissions from all UEs (User Equipments) contribute to the interference level and the Rise over Thermal (RoT) in uplink. However, there are scenarios where the HS-DPCCH transmissions could be reduced for lowering the RoT and enabling higher system throughput capacity or increasing coverage.
In one HS-DPCCH sub-frame, the Channel-Quality Indicator (CQI) is transmitted in the 2nd and 3rd slots, and is the key indicator for HSDPA (High-Speed Downlink Packet Access) down-link channel quality and its frequent reporting is essential to maintain a good DL (downlink) throughput.
The CQI is signaled from UE to Node B on one or several uplink HS-DPCCH physical channels, together with other channel state information, depending on configured downlink transmission mode.
FIG. 1 shows the messages exchanged between Node-B and the UE during typical data call set up when the UE is configured with only one carrier. From the common pilot channels, P-CPICH and S-CPICHs when needed, UE estimates the channel, computes the channel quality information and pre-codes channel indicator. This information along with hybrid ARQ (Automated Repeat-reQuest) ACK/NAK (Acknowledgement/Negative acknowledgement) is reported to Node-B using dedicated physical control channel, HS-DPCCH. The structure of HS-DPCCH for a single carrier is shown in FIG. 2 when the UE is configured in non MIMO (Multiple-Input Multiple-Output) mode. It can be seen from FIG. 2 that in the first slot HARQ (Hybrid ARQ) ACK is transmitted (10 bits) and in the 2nd and 3rd slots CQI information is transmitted (20 bits). Note that CQI of 5 bits is block encoded (Reed Muller code) to form 20 bits.
Once the Node-B receives this information, it allocates the required channelization codes, modulation and coding to the UE after scheduling. This information is conveyed to UE by high-speed shared control channel (HS-SCCH), see FIG. 1. Once the UE detects the HS-SCCH, downlink transmission starts through data traffic channel using High-Speed Physical Downlink Shared Channel (HS-PDSCH).
According to the CSI (Channel State Information) reduction technique, the RNC (Radio Network Controller) configures the UE with two reporting periods, the primary and the secondary, during the RRC (Radio Resource Control) configuration. For example the RNC configures a low value for frequent reporting (normal or primary) and a high value for non-frequent reporting (secondary).
With reference to FIG. 3, at first the UE reports the CQI with the primary reporting period, i.e. it operates in a high reporting cycle. However, during say N successive Transmission Time Intervals (TTI's) if it is not scheduled then the UE assumes that there is no data to be scheduled in the downlink direction on that carrier and goes to the secondary reporting period for CSI, i.e. the UE operates in a low reporting cycle. Note that the UE decodes the downlink control channel (HS-SCCH) to identify whether it is scheduled or not. The value of N is configured by the RNC during the data call setup.
When the UE is reporting CSI using secondary reporting period, whenever it receives HS-SCCH correctly, it will move to primary reporting period starting from that TTI or with some reference TTI.
Configuring the timer value, N, is a tricky problem. If the timer value is very high, then the UE does not move to the higher cycle lengths even though there is no data to be transmitted in the downlink direction. If the timer value is very small, the UE switches between low reporting cycle and high reporting cycle often.
For example, let's say the Node B schedules the UE and send this information using HS-SCCH and say the UE didn't receive this (CRC (Cyclic Redundancy Check) failure), then if the timer value is very small, it will move to the higher reporting cycle. Since the Node B does not know that the UE moved to higher reporting cycle it assumes that the UE is reporting HS-DPCCH in low reporting cycle and tries to detect during these periods. If the UE is scheduled with the random CQIs there is a high probability that these packets will be a failure and decrease in the UE throughput.